1. Field of the Invention
The present invention describes a process for the preparation of basic (meth)acrylates (IV) in high purity and with a high yield by transesterification of an industrial lower alkyl (meth)acrylate I with basic alcohols (R2OH).
2. Description of the Background
High purity is understood as meaning a purity of at least 99.8%, the total content of saturated impurities (without water) being not more than 1000 ppm and the content of N,N′-dimethylpiperazine, ethylene glycol di(meth)acrylate and vinyloxyethyl (meth)acrylate being not more than 100 ppm in each case.
(Meth)acrylates are useful starting compounds for the preparation of polymers and copolymers which are used, for example, as finishes, dispersions or adhesives.
Saturated impurities, i.e. those which have no carbon-carbon multiple bonds, e.g. alcohols, ethers and acetic and propionic acid derivatives, are disadvantageous in that they withstand the polymerization unchanged, i.e. are not incorporated into the polymer, and may lead to the product having an annoying odor. In order to separate off the residual volatile components, for example from dispersions, expensive treatments (deodorizations) are additionally required.
As a rule, a treatment referred to as physical deodorization is carried out and comprises stripping the dispersion with steam, air or nitrogen in a stirred container (German Published Application DAS 12 48 943) or in a counter current column. The treatment is carried out in one or more stages, depending on the amount and the boiling points of the components to be separated off. The removal of these impurities is accordingly an expensive procedure, which moreover cannot be carried out in the case of heat-sensitive dispersions owing to the thermal stress.
The content of ether (dibutyl ether is entrained, for example, with butyl acrylate as lower (meth)acrylate I, see below) also has an adverse effect if the procedure is carried out in the presence of oxygen-containing gases, such as air, for example for stabilization. It is known that, in the presence of oxygen, ethers very readily form peroxides and it is known that these may then initiate a polymerization of (meth)acrylate compounds, which may even take place explosively.
The ether content is accordingly not only a quality problem but also a safety problem.
Ethylene glycol di(meth)acrylate and vinyloxyethyl (meth)acrylate, which occur as secondary components in the preparation of alkylaminoethyl (meth)acrylates, contain two unsaturated groups and therefore act as crosslinking agents in the polymerization. This is extremely disadvantageous since consequently the polymerization and the quality of the polymers are adversely affected, for example by gel formation. In addition, they influence the shelf-life.
The preparation of basic (meth)acrylates IV by transesterification of lower (meth)acrylates I with basic alcohols R2OH is generally known.
It is furthermore generally known that the transesterification is an equilibrium reaction. In order to achieve economical conversions, the resulting lower alkanol R1OH, being the component with the lowest boiling point, is therefore generally removed continuously from the equilibrium by distillation, a very pure alkanol fraction being desirable for economic reasons in order to be able to use it again, for example in the preparation of the lower (meth)acrylate I by esterification. Owing to the position of the boiling points and formation of azeotropic mixtures, however, this distillate generally does not consist of pure lower alkanol R1OH but is contaminated with the lower (meth)acrylate I and possibly with basic alcohol R2OH.
Since, for economic reasons, it is expedient to utilize the distillate, impurities have an adverse effect, particularly when they are basic impurities, i.e. compounds having an amino group.
The particularly economical recycling to the synthesis of the lower ester is especially influenced thereby, cf. for example EP-A2 906 902, page 3, lines 4-16.
EP-A2 906 902 describes a process for the preparation of alkylamino (meth)acrylates by transesterification of alkyl (meth)acrylates with alkylaminoalcohols in the presence of a catalyst, e.g. dibutyltin oxide, in which the alcohol-containing distillate (azeotropic mixture) is passed, either directly or after a further distillation, over an acidic ion exchange resin. The basic nitrogen-containing impurities from the distillate are bound by the acidic groups and thus separated from the alkanol/(meth)acrylate mixture, which can then be used again in the synthesis of the lower (meth)acrylate. The working-up of the transesterification mixture is carried out in a plurality of distillation stages, the additional formation of the Michael adducts during the catalyst removal being reduced as far as possible.
Michael adducts are defined as the compounds formed by addition of alcohols at the double bond of the (meth)acrylates.
It is generally known that this addition (cf. equation I) takes place in particular in the presence of alkaline catalysts (Organikum, 17th Edition, page 506, VEB Deutscher Verlag der Wissenschaften, Berlin 1988).

By means of a two-stage catalyst removal (EP-A2 906 902, page 4, lines 51-57), the additional formation (ratio of increase) of the Michael adducts is kept below 2%. According to examples III-1, III-2 and III-3, the distillation temperatures and the residence times play a decisive role. On increasing the temperature in stage 2 or the residence time in both stages, the ratio of increase of the Michael adducts does in fact increase substantially (from 0.48 to 0.96 and 2.2%, respectively).
The actual, absolute content of the Michael adducts in the reaction mixture, which is decisive for the yield and the cost-efficiency of the process, is mentioned nowhere in EP-A 906 902.
The process has the following disadvantages:    1. Necessity of purification using an ion exchanger.    2. The regeneration and disposal of the exchange resin laden with the basic impurities is expensive and environmentally polluting.    3. It requires from 5 to 7 distillation steps and is thus technically complicated.    4. The yield is low (about 33%, example III-1).    5. The amino alcohol must be metered in continuously over a long period (4 hours, cf. example III-1) in order to reduce the formation of the Michael products.    6. Long reaction times are required (7-8 hours) which reduces the cost-efficiency.
Our own investigations have shown that in particular the reaction time (i.e. the residence time in the reactors) has a decisive effect on the formation of the Michael adducts (cf. example 3). On the other hand, the temperature and the residence time in the catalyst removal surprisingly are of no importance for the formation of the Michael adducts in the novel process (cf. comparative examples 1 and 2).
As a rule, methyl and ethyl (meth)acrylate are used as starting materials in the transesterification (EP-A 960 877, FR 2 617 840), whereas butyl (meth)acrylate is regarded as being disadvantageous owing to its high boiling point (U.S. Pat. No. 2,832,800, column 2, lines 60-70).
Since the literature on the transesterification of alkyl (meth)acrylates provides no detailed information about the accompanying substances and impurities in the starting esters used, it must be assumed that the purity of the (meth)acrylates used is very high and no troublesome components are present.
However, the use of esters of high purity is disadvantageous since they have to be purified in a technically complicated manner by distillation after their preparation. In view of the generally known high tendency of (meth)acrylate compounds to polymerize under thermal stress, this is particularly disadvantageous.
In particular, titanium alcoholates whose alkyl groups are C1- to C4-alkyl radicals, e.g. tetramethyl, tetraethyl, tetraisopropyl, tetrapropyl, tetraisobutyl and tetrabutyl titanates, are proposed as catalysts for the preparation of (meth)acrylates by transesterification (cf. for example EP-B1 298 867, EP-A2 960 877). Inter alia, titanium phenolates (German Laid-Open Application DOS 200 86 18), metal chelate compounds of, for example, hafnium, titanium, zirconium or calcium, alkali metal and magnesium alcoholates, organic tin compounds or calcium and lithium compounds, for example oxides, hydroxides, carbonates or halides thereof, are furthermore proposed as catalysts.
For economic and ecological reasons, in particular alkyl titanates are used, although they are, for example, sensitive to even traces of water and some titanium alcoholates are unstable at relatively high temperatures. The result is fouling (see below) of the apparatus walls.
Furthermore, it is generally known that alkyl titanates promote the polymerization of (meth)acrylates and may therefore give rise to the formation of polymer during the transesterification and the working-up of the transesterification mixture (German Laid-Open Application DOS 20 08 618, page 3, German patent 1,067,806, column 1, lines 39-41).
Another disadvantage is that, because their activity is relatively low in some cases, titanium alcoholates necessitate high transesterification temperatures in order to achieve economical conversions or reaction times (EP-A 160 427, page 2, lines 23-32). This in turn can lead to increased polymer formation and fouling.
Another problem is the loss of activity suffered by titanium alcoholates in the course of time (German Laid-Open Application DOS 28 05 702, page 5, lines 12-21). In order to achieve economical conversions, the amount of catalyst must be increased and/or the reaction time lengthened. In view of the instability of the titanates and the byproduct and polymer formation, this is known to be disadvantageous.
Added to this is the fact that (meth)acrylic acid compounds have a considerable tendency to polymerization, very particularly if heat acts on them. Especially on the preparation and the distillative purification, they are exposed to temperatures which can easily initiate an undesired polymerization. The use of polymerization inhibitors, as is generally recommended, also cannot completely prevent the polymer formation.
Soiling of the apparatuses, blockage of pipes and pumps and fouling of column trays and heat exchanger surfaces are as a rule the result of polymer formation. The cleaning of the plants is a complicated, expensive and environmentally polluting process, and the yield and the availability of the plants (run time) are also greatly reduced as a result.
JP-A 3-112 949 describes a process for the preparation of dimethylaminoethyl acrylate by transesterification of n-butyl acrylate with dimethylaminoethanol using tetra-n-butyl titanate, in which the purification of the reaction mixture is effected in the absence of oxygen.
However, the yields of less than 90% are a disadvantage of this process.